Federer, playing in his first tournament since losing in the Indian Wells final nearly four weeks ago, made too many mistakes in a match featuring some spectacular shots.

He initially moved 3-1 ahead via an early break but Monfils hit straight back and then outplayed the Swiss from the baseline to break again at 4-4 before clinching the opening set. The second went with serve, then Federer built a 5-3 lead in the tie-break before losing four points in a row.

“It was not the best end to the match,” Federer said. “I should have done better with the break up in the first. I think I did a good job to hang around the second set. I also had chances of my own. But I never felt like things were really happening the way I wanted them to be going during the whole match.”

Federer said he would return to Switzerland to prepare for more clay-court action in Istanbul and Madrid.

Wawrinka also looked like he needs more practice. The defending champion hit 41 unforced errors as he went down to Grigor Dimitrov, the ninth seed, in under an hour, putting up only token resistance as his game fell apart in a 6-1, 6-2 rout.

Dimitrov will now face Monfils in the quarter-finals.

The third seed Rafa Nadal, who ruled in Monte Carlo from 2005-12, overcame John Isner, but was given a fright.

The Spaniard had to save two set points in the opening set and then found himself pegged back before advancing to the last eight 7-6, 4-6, 6-3.

Nadal broke decisively in the fourth game of the third set to set up a meeting with the Spanish fifth seed David Ferrer or Frenchman Gilles Simon.